1. Field of the Invention
The present invention relates to a laser beam generating apparatus, and more particularly to a laser beam generating apparatus wherein a resonator, provided outside of a laser oscillator, contains a barium borate crystal and a laser beam in the ultraviolet region is supplied, with harmonic content extracted from the laser beam generated by the laser oscillator. In further detail, the invention relates to an optical system for irradiating optical components with an ultraviolet beam of not more than 400 nm in wavelength or a laser beam generating apparatus for generating an ultraviolet beam of not more than 400 nm in wavelength.
2. Description of the Related Art
If, in the field of semiconductor manufacturing for example, a laser beam in the ultraviolet region can be used in a stepper (a sequentially shifting exposure system), finer processing than what is currently done will be made possible, enabling large-capacity memory elements which are further enhanced in the level of integration to be manufactured. A laser beam in the ultraviolet region can be applied not only for this purpose but also to photochemical reactions and biotechnology, and therefore practical availability of ultraviolet lasers in many different fields is awaited.
By a method according to the related art with high potential for practical application to generate a laser beam in the ultraviolet region, a barium borate crystal, which is a nonlinear optical crystal, is disposed in a resonator provided outside the laser oscillator, and secondary harmonic content is extracted from the laser beam generator by the laser oscillator.
Where a laser beam in the ultraviolet region is to be generated by this method, a harmonic content of the required intensity, i.e. an ultraviolet laser beam, is obtained by squeezing the waist of the laser beam (i.e. the radius of the cross section of the beam) which is allowed to pass the barium borate crystal, because the nonlinear conversion coefficient of the barium borate crystal is generally small.
However, the squeezing of the waist of the laser beam results in a greater power density of the laser beam in the barium borate crystal, leading to the problem that they may be heavily damaged both on the surface and inside.
Therefore, such a laser beam generating apparatus according to the related art, although an ultraviolet laser beam is obtained in a high output during the early phase of its use, steeply drops in output with the lapse of time, making it difficult for a high output to be maintained for a long period.
Incidentally, by the conventional method, if the power of an ultraviolet laser beam is 100 mW, the output can last for not more than 400 hours, and the velocity of degradation (the velocity of power drop) is about 1.35xc3x9710xe2x88x924 [%/hour].
The damage to the barium borate crystal can be more clearly observed by microscope. FIG.
FIG. 1 is a schematic diagram showing the result of microscopic observation of the trace of a beam pattern formed in a barium borate crystal where the beam waist is 23 xcexcm.
This diagram is a front view of the laser beam emitting end face of the barium borate crystal, in which the area surrounded by a dotted line 102 is the part damaged by the laser beam, looking more turbid than the surrounding normal part. Incidentally, it is because the generated harmonic content spreads at an angle of about 4xc2x0 to the original laser beam that the damage is oblong laterally.
Furthermore, there is another problem that optical components deteriorate in performance characteristics when irradiated in the atmosphere with an ultraviolet ray of not more than 400 nm in wavelength, presumably because the optical losses of the optical components increase in such a situation. Such optical losses are presumed to occur as moisture and oily contents in the atmosphere on the surface of the optical components react and the reaction products and particles around them stick to the surface of the optical components.
When an ultraviolet beam of not more than 400 nm in wavelength is to be generated, in wavelength conversion using an external resonator (for information on which, see M. Oka and S. Kubota, Jpn. J. AppI. Phys. Vol. 31 (1992), pp. 513, and M. Oka et. al., in the Digest of Conference on Laser and Electro-Optics (OSA, Washington, D.C., 1992), paper CWQ7) or the like, the harmonic output is significantly reduced by intricate performance deterioration of a mirror or a nonlinear optical element arranged within the external resonator. This deterioration again, as the present inventor sees it, seems attributable to similar circumstances to what was described above. When, for instance, an ultraviolet beam of not more than 400 nm in wavelength formed by wavelength conversion passes an optical component, such as a mirror, it adversely affects the performance of the optical component (e.g. the mirror).
Therefore, for use where optical components are to be irradiated with an ultraviolet beam of not more than 400 nm in wavelength as well as where an ultraviolet beam of not more than 400 nm is to be generated, there is a keen call for the development of an optical system which can prevent the optical performance of optical components from being adversely affected by an increase in optical losses or their output performance and other attributes from being deteriorated.
Problems with the aforementioned related art will be described below with reference to drawings. For instance, where a dominant wave of 532 nm in wavelength is to be converted in wavelength into an ultraviolet beam of 266 nm in wavelength by using an external resonator, the structure of the external resonatorxe2x80x94art will be as illustrated in FIG. 2.
In FIG. 2, what are denoted by reference numerals 10, 12 and 14 are highly reflective mirrors having an ultra-high reflectance at a wave-Length of 532 nm, e.g. a reflectance of 99. 95% or more; what is denoted by numeral 8 is an incidence mirror having a high reflectance, e.g. a reflectance of 99% at a, wavelength of 532 nm; and what is denoted by numeral 6 is a nonlinear optical crystal BBO, which is a wavelength converting element coated with a less reflective film having a low reflectance, e.g. a reflectance of not more than 0.1% at a wavelength of 532 nm., The highly reflective mirror 14 is installed over a VCM (see the above-cited SRF92 collection of preliminary papers), which is a positioning device (not shown), and can be controlled by, for instance, a servo drive system. The elements 6, B, 10, 12 and 24 referred to above constitute an external resonator section.
When a dominant Wave (of 532 nm in wavelength here) schematically indicated by an arrow 30 in FIG. 2 is brought to incidence on this external resonator, it is amplified between the mirrors, and the amplified dominant wave is converted by the nonlinear optical crystal 6 (BBO) into a secondary harmonic (of 266 nm in wavelength here). This secondary harmonic is schematically indicated by an arrow 31 in FIG. 2.
When such a wavelength conversion as described above is accomplished in the atmosphere, optical losses (to be specific, mainly scattering) of the mirrors (especially the mirror 10) increase. The relationship between an optical loss and the power of the dominant wave of 532 nm in wavelength, amplified in the external resonator, can be represented by the following equation.
Pxcfx89={square root over ( )}(xcex4cav2+4xcex3SHPixe2x88x92xcex4cav)2xcex3SHxe2x80x83xe2x80x83Equation 1
Where xcex4cav is the optical loss at a wavelength of 532=in the external resonator; Pxcfx89, the power of the amplified dominant wave; Pi, the power of the dominant wave of 532 nm in wavelength coming incident on the external resonator; and xcex3SH, a constant known as a nonlinear conversion factor determined by the crystalline length of the nonlinear optical crystal 6 (BBO), wavelength of the dominant wave, spot size and focusing parameter.
Equation 1 given above reveals that, in the external resonator, the power Pxcfx89 of the dominant wave decreases with an increase in the optical loss xcex4cav.
On the other hand, the relationship between the power of the dominant wave and that of the secondary harmonic can be represented by Equation 2 below.
P2xcfx89=xcex3SHPxcfx892xe2x80x83xe2x80x83Equation 2
Where Pxcfx89 is the power of the dominant wave coming incident on the nonlinear optical crystal 6 (BBO); P2xcfx89 the power of the secondary harmonic generated by wavelength conversion by the nonlinear optical crystal 6 (BBO); and xcex3SH, said nonlinear conversion factor.
Equation 2 given above reveals that, when the power Pxcfx89 of the dominant wave decreases, the power P2xcfx89 of the secondary harmonic also decreases. In a rough measure, the power of the secondary harmonic halves in about 5 to 10 hours.
An object of the present invention is to provide a laser beam generating apparatus capable of generating a laser beam in the ultraviolet region at a high power level for a long period of time.
In order to achieve the above-stated object, according to the invention, there is provided a laser beam generating apparatus comprising a laser oscillator; an external resonator on which a laser beam emitted from said laser oscillator comes incident; and a barium borate crystal disposed on an optical path within said external resonator, whereby a harmonic content is extracted from said laser beam emitted from said laser oscillator to supply a laser beam in the ultraviolet region, having a configuration in which the length of said barium borate crystal along said optical path is within the range of 2 mm to 6 mm and the beam waist of said laser beam passing said barium borate crystal in the position of said barium borate crystal is within the range of 40 xcexcm to 60 xcexcm.
According to the invention, there is also provided a laser beam generating apparatus comprising a laser oscillator; an external resonator on which a laser beam emitted from said laser oscillator comes incident; and a barium borate crystal disposed on an optical path within said external resonator, whereby a harmonic content is extracted from said laser beam emitted from said laser oscillator to supply a laser beam in the ultraviolet region, having a configuration in which the length of said barium borate crystal along said optical path is greater than 6 mm and the beam waist of said laser beam passing said barium borate crystal in the position of said barium borate crystal is greater than 60 xcexcm.
A laser beam generating apparatus according to the invention, having a configuration in which the length of the barium borate crystal along the optical path is within the range of 2 mm to 6 mm and the beam waist of said laser beam passing said barium borate crystal is within the range of 40 xcexcm to 60 xcexcm, and the power density of the laser beam in the barium borate crystal is thereby prevented from becoming greater than necessary, is increased in the service life of the barium borate crystal and enabled to generate an ultraviolet laser beam for a long period of time at a high output.
Further, a laser beam generating apparatus according to the invention, having a configuration in which the length of the barium borate crystal along the optical path is greater than 6 mm and the beam waist of the laser beam passing the barium borate crystal is greater than 60 xcexcm, and the power density of the laser beam in the barium borate crystal is thereby prevented from becoming greater than necessary, is increased in the service life of the barium borate crystal and enabled to generate an ultraviolet laser beam for a long period of time at a high output.
In order to achieve the above-stated object, in an optical system irradiated with an ultraviolet beam according to the invention, optical components are irradiated with an ultraviolet beam of not more than 400 nm in wavelength, and 99.9% or more of the ambience of the optical components is nitrogen.
In another such optical system, 99.9% or more of the ambience of the optical components is air.
In still another such optical system, the ambience of the optical components is a gas whose moisture content is not more than 0.1%.
In yet another such optical system, the ambience of the optical components is a gas whose hydrocarbon compound content is not more than 0.1%.
In order to achieve the above-stated object, in a laser beam generating apparatus according to the invention, where an ultraviolet beam of not more than 400 nm in wavelength is to be generated by wavelength conversion with a nonlinear optical crystal disposed in an external resonator, 99.9% or more of the ambiance of its mirror section and nonlinear optical crystal section is nitrogen.
In another such laser beam generating apparatus, 99.9% or more of the ambiance of its mirror section and nonlinear optical crystal section is air.
In still another such laser beam generating apparatus, the ambiance of its mirror section and nonlinear optical crystal section is a gas whose moisture content is not more than 0.1%.
In yet another such laser beam generating apparatus, the ambiance of its mirror section and nonlinear optical crystal section is a gas whose hydrocarbon compound content is not more than 0.1%.
In another such laser beam generating apparatus, the ambiance of its mirror section and nonlinear optical crystal section is a gas whose moisture content and hydrocarbon compound content is not more than 0.1%. At the same time, 1% or more of the ambiance of its mirror section and nonlinear optical crystal is oxygen. The above-mentioned value of ratios is based on volume percentage.
The present invention is a result of various studies taking note of the ambiance in which irradiation with an ultraviolet beam of not more than 400 nm in wavelength is done, or such an ultraviolet beam is generated, an aspect which had not been considered previously, especially with respect to the purity of nitrogen or air or the proportion of the moisture or oily (hydrocarbon compound) content. According to the invention, an ultraviolet beam of not more than 400 nm in wavelength can give a satisfactory result for the object of the invention as long as the purity of nitrogen or air or the proportion of the moisture or oily (hydrocarbon compound) content is within the applicable range envisaged according to the invention.
Incidentally, although the Gazette of the Japanese Patent Laid-open No. Sho 60-57695 discloses a technique to seal in a laser element airtightly and thereby prevent its deterioration, that of the Japanese Patent Laid-open No. Hei 4-84481 discloses a technique to protect a laser element by enclosing inert gas in the package of a semiconductor laser apparatus, and that of the Japanese Patent Laid-open No. Hei 5-110174 discloses a technique to use inert gas as the ambiance of a laser diode, but none of these disclosures concerns a configuration similar to the present invention.